In the fabrication of LEDs, the adoption of patterned sapphire substrates (hereinafter referred to as PSS) can help to achieve reduced internal light loss, enhanced lateral light extraction, less heat generation, extended life spans and a much milder lattice mismatch between sapphire and gallium nitride (GaN) of the LEDs. Therefore, the existing PSS technologies have been widely used in the LED industry. Due to the demanding PSS requirements for pattern uniformity, steppers are usually used for the exposure of the patterns.
At present, most common PSS patterns are arranged in a 60°-, 120°- or 180°-array, as shown in FIG. 1. However, steppers often perform exposure orthogonally in an X-Y coordinate system. Therefore, photomasks adopted in steppers usually assume a rectangular shape with chrome-plated opaque areas 01 that do not allow the passage of light from the light source and a transparent area 02 that is not plated with chrome and allows light from the light source to transmit through the transparent area 02. The photomask has a rectangular shape and the four sides thereof constitute boundaries of the photomask. As shown in the figure, some opaque areas 01 are cut by the boundaries of the photomask into halves. Generally, the round opaque areas 01 have a diameter of 2-2.3 μm and a minimum allowable pitch of 3 μm. With such a conventional pattern, an optical proximity effect (OPE) may be given rise due to excessively narrow gaps between the boundaries of the photomask and the nearby round opaque areas 01.
If P represents a recurrence period of the round opaque areas 01, D represents the diameter of the round opaque areas 01, and a represents the pitch between adjacent round opaque areas 01, then a=P−D. In the pattern of FIG. 1, a boundary of the photomask may traverse gaps that define the pitch a, so that the boundary is spaced apart from the nearest round opaque areas 01 by a distance of only a/2. Generally, P=3 μm and D ranges from 2 μm to 2.3 μm, so a/2 ranges from 0.35 μm to 0.5 μm. On the other hand, a photolithography machine has a lower resolution limit given by
      R    =                  k        1            ×              λ        NA              ,where λ denotes the wavelength, usually of 365 nm, NA denotes the numerical aperture, usually in the range of 0.32 to 0.5, and k1 denotes the process factor, usually of 0.7 μm. It will be found from a calculation that the lower resolution limit of photolithography machines commonly used in PSS technology is in the range of 0.5 μm to 0.8 μm, greater than a/2. In other words, the distances between the boundaries and adjacent round opaque areas 01 are too small. FIG. 2 shows a light intensity distribution of the light incident on the surface of a photoresist layer when using the photomask with the pattern of FIG. 1 to expose the photoresist. In FIG. 2, the photomask is shown to be placed in an X-Y coordinate system, the dark areas correspond to the opaque areas 01 and the bright area corresponds to the transparent area 02. An actual measurement revealed that the light intensity at the boundaries of the photomask was 50% of the light intensity at the central transparent area 02 of the photomask. This is considered to be a consequence of an optical proximity effect (OPE). Due to the optical proximity effect, in the image formed by exposing the photomask, those portions of the image corresponding to the round opaque areas 01 close to the boundaries of the photomask often present dimensional unevenness or cannot be clearly developed.
In order to overcome this problem, Chinese Patent Publication No. CN102520576B of Chinese Patent Application No. CN201110367148.5, filed Jun. 27, 2012, proposes a PSS photomask pattern, as shown in FIG. 3, which is made up of hexagonal units. However, in this pattern, a portion of the transparent area between a round opaque area and a boundary of the photomask has a width that is only one half of a distance between adjacent round areas, and the corresponding light intensity distribution is shown in FIG. 4. An actual measurement also revealed that the light intensity at the boundaries of the photomask was 50% of the light intensity at the central area of the photomask. Therefore, this solution fails to solve the aforementioned problem.
Chinese Patent Publication No. CN103365070A of Chinese Patent Application No. CN201210089168.5, filed Oct. 23, 2013, describes a photomask pattern consisting of two sets of holes that are periodically staggered and configured to produce light beams with different phases. While this pattern can produce a clear image, it requires the use of negative photoresist, and accordingly, proper adjustments must be made in the photolithography system, so the process cost is high.
Chinese Patent Publication No. CN103337566A of Chinese Patent Application No. CN201310243141.1, filed Oct. 2, 2013, proposes a method including the steps of: forming a patterned photoresist corresponding to part of a desired substrate pattern on a front side of a sapphire substrate using a photolithography process; depositing a stop layer over the front side of the substrate; removing the photoresist by a lift-off process, such that only the portion of the stop layer on the substrate is retained; forming another patterned photoresist corresponding to the remaining part of the desired substrate pattern by another photolithography process; depositing another stop layer over the substrate; removing the photoresist and the portion of the stop layer thereon using another lift-off process; and etching the sapphire substrate to form the desired substrate pattern by a wet or dry etching process. Although this solution can effectively mitigate the foregoing problem, the process is complex and costs much.
Chinese Patent Publication No. CN103576440A of Chinese Patent Application No. CN201310473082.7, filed Feb. 12, 2014, proposes to modify the opaque areas into plum-like shapes. It has also been proposed in the prior art to use volcano-shaped opaque areas. However, those opaque areas can only improve the light extraction, but still fail to solve the problem of blurred image edges arising from excessively narrow gaps between photomask boundaries and opaque areas.
Therefore, there is a need for a PSS photomask pattern and an exposure method, which can solve the problem of blurred pattern image edges that may arise from too small distances between photomask boundaries and opaque areas while not increasing process complexity and cost.